Posted
by
kdawsonon Friday July 24, 2009 @01:43PM
from the foaming-brine dept.

BuzzSkyline writes "It's possible to produce energy by simply mixing fresh and salty water. Although chemists and physicists have long known about the untapped energy available where fresh water rivers pour into salty oceans — it's equivalent to 'each river in the world ending at its mouth in a waterfall 225 meters [739 feet] high' — the technology for exploiting the effect has been lacking. An Italian physicist seems to have solved the problem with the experimental demonstration of a 'salination cell' that creates power given nothing more than input sources of salty and fresh water. The researcher believes that this renewable, environmentally friendly energy source could be deployed in coastal areas and could provide another addition to the green-tech roster. A paper describing the technology is due to be published in an upcoming issue of the journal Physical Review Letters."

Nice to see somebody talking about energy from water salinization once in a while, but that is not the first experiment to gather a few microjoules at lab. Up to now, no aparatus could be scaled up, all of them hit that "we just need better materials" barrier. There is a reason for that, because of the way difusion works, each device can create at most 100mV, and that will fall almost exponentially down to near 10mV once one starts gathering more than 5% of the available energy.

Just put that on the right perspective, there are just a few specialized diodes that will dissipate less than 100mV on the charge going through it. A normal silicon diode will dissipate 700mV, and there is simply no diode that will dissipate less than 10mV. Also, to get some sane amount of power at 10mV one needs quite a big current, the charge is available to extract that current, but the resistence of your circuit (and the capacitor's dieletric is a piece of the circuit) is a huge barrier. To create 1kW, one'd need a total current of 10^5A (of ions flowing into and out of the coal, if not electrons flowiong throug the circuit), with a total resistence of 10^-7 ohms. To reach such small reistences it is normaly needed lots and lots of material, or "just" better material.

It's osmotic pressure. You have salty water and pure water, and there's a force produced when they contact, because the ions in the saltier water are driven by entropy into the less-salty water.
The energy you're stealing is solar power: the sun heats the salty water, evaporating out pure water, that goes up into the clouds and then rains, forming the rivers of pure water.
This is just a convoluted solar power system. But then again, so is everything else: wind, gravity, and more distantly, nuclear and oil.
The main environmental issue would be interfering with fish migration, for the many (very economically valuable) fish that live in the sea but spawn in rivers, like salmon. Which, by the way, are near miracles from a biochemistry standpoint, since they live part of their lives in the sea, where they're fighting to keep those same ions out of themselves because sea water has about twice the ion concentration as animal tissue so they have to maintain a more pure internal environment, and then they swim into fresh water, where they have to fight to keep from bleeding all their ions out, since many streams have about 1/2 or less the ion concentration as animal tissue. There aren't that many animals that can manage it.

Okay, what this means is that the energy contained in a glass of fresh water is higher than the energy of the same amount of water with salt in it.

Every time you add salt to a glass of water, the temperature of the water increases. Imperceptibly, to most people, but the water actually heats up. That experiment is usually a lab in a physical chemistry/thermodynamics class.

Where does the energy come from? The fact that having salt in the water is more stable than not having salt in the water. We could actually explain THAT, but then there have to be terms like "configurational entropy" involved.

The methodology for the cell that actually converts this "mixing energy" is well beyond me. It has something to do with electrostatics related to this entropy of mixing. It's not explained in TFA because TFA-writer probably didn't understand it either. You could go to the original paper, maybe. If you have access.

So if a car that's made of salt and a car that's made of water crash into each other....

As such, any time we find a new source of power, you can damn well bet nature has gotten there first, and that our exploitation of said power will have negative consequences for the species already using it.

This sentiment of yours is dangerous in the sense that it is wrong yet rational enough that too many people could believe it.

Nothing was using the energy stored in uranium or oil until we got around to using it. And neither us nor any other creature is harnessing e.g. the energy of deuterium and tritium contained in seawater. Nothing is even using the energy of the sun shining on the desert.

Another problem with your idea: energy cannot be really "used", it can only be directed elsewhere. Sooner or later every form of energy will change into heat. We cannot stop this, but before it takes place we can transform energy into other forms to do something useful. Example: when the sun shines on the desert, it is converted to heat straight away. But when we put solar panels there, we can redirect a part of the energy to our homes and use the energy from the sun there, where in the end it will also be turned into heat.

I could go on about how humans are not artificial, but part of nature, but the main premise of your post is already invalidated so I'll stop.

That wont bother PETA at all. They have nothing against killing animals senselessly. They only get angry if you try to justify the death of the animal by using its fur or meat for something useful, but if you just throw it all out it's all good with them. http://www.petakillsanimals.com/ [petakillsanimals.com]

In super-layman terms:When you put noodles in hot water, they swell. They want to replace noodles with salt water and capture the energy of swelling.

In slightly less layman terms:Recall the principle of induction charging: you hold a grounded metal plate next to a charged one, disconnect the ground, and then remove the charged plate. Both plates are now charged, even though in the beginning one of them was grounded. The effect exploited in the device is similar, except they use the higher concentration ions in the salty water as the 'charged plate' and flushing with less salty water as the equivalent of 'removing the charged plate'.